This invention relates in general to method for balancing articles for rotation. In particular, this invention relates to an improved method for statistically analyzing the operation of a machine for balancing articles for rotation so as to reduce measurement errors and thereby enhance productivity and quality.
Drive train systems are widely used for generating power from a source and for transferring such power from the source to a driven mechanism. Frequently, the source generates rotational power, and such rotational power is transferred from the source to a rotatably driven mechanism. For example, in most land vehicles in use today, an engine/transmission assembly generates rotational power, and such rotational power is transferred from an output shaft of the engine/transmission assembly through a driveshaft to an input shaft of an axle assembly so as to rotatably drive the wheels of the vehicle. To accomplish this, a typical driveshaft includes a hollow cylindrical driveshaft tube having a pair of end fittings, such as a pair of tube yokes, secured to the front and rear ends thereof. The front end fitting forms a portion of a front universal joint that connects the output shaft of the engine/transmission assembly to the front end of the driveshaft tube. Similarly, the rear end fitting forms a portion of a rear universal joint that connects the rear end of the driveshaft tube to the input shaft of the axle assembly. The front and rear universal joints provide a rotational driving connection from the output shaft of the engine/transmission assembly through the driveshaft tube to the input shaft of the axle assembly, while accommodating a limited amount of angular misalignment between the rotational axes of these three shafts.
Ideally, the driveshaft tube would be formed in the shape of a cylinder that is absolutely round, absolutely straight, and has an absolutely uniform wall thickness. Such a perfectly shaped driveshaft tube would be precisely balanced for rotation and, therefore, would not generate any undesirable noise or vibration during use. In actual practice, however, the driveshaft tube and other components of the driveshaft usually contain variations in roundness, straightness, and wall thickness that result in minor imbalances when rotated at high speeds. To prevent such imbalances from generating undesirable noise or vibration when rotated during use, therefore, it is commonplace to counteract such imbalances by performing a corrective action, such as by such as by securing one or more balance weights to the driveshaft or by removing material therefrom, for example. The corrective action is taken to counterbalance the imbalances of the driveshaft such that it is balanced for rotation during use.
Traditionally, the balancing process has been performed with the use of a conventional balancing machine. Referring to FIG. 1, a typical balancing machine, indicated generally at 10, includes a pair of fittings 12 that are adapted to support the ends of a driveshaft 14 thereon. The balancing apparatus 10 further includes a motor (not shown) for rotating the driveshaft 14 at a predetermined speed. As the driveshaft 14 is rotated, the balancing machine 10 senses vibrations that are caused by imbalances in the structure of the driveshaft 14. The balancing machine 10 is responsive to such vibrations for determining (1) if the driveshaft 14 is out of balance and, if so, (2) the magnitude and location of a corrective action that can be taken to counterbalance the imbalances of the driveshaft 14 such that it is balanced for rotation during use. The addition of a balancing weight 16 at the identified location and having a mass of the identified magnitude is an example of such a corrective action. The rotation of the driveshaft 14 is then stopped to allow such corrective action to be taken. Then, the driveshaft 14 is again rotated to confirm whether proper balance has been achieved or to determine if additional corrective action required. A number of such balancing machines of this general structure and method of operation are known in the art.
Ideally, each driveshaft 14 would be supported on the balancing machine 10 and rotated and measured only once to confirm that it was manufactured in such a manner as to be properly balanced. As a practical matter, however, driveshaft assemblies are not manufacture so precisely. Thus, each driveshaft 14 is usually supported on the balancing machine 10 and rotated and measured at least twice, a first time to measure the magnitude and location of the imbalance therein, and a second time to confirm that proper balance has been achieved after the corrective action has been taken.
Although known balancing machines of this general type have been effective, this balancing process is further complicated by the fact that conventional balancing machines are subject to measurement errors that result merely from the use thereof. Such measurement errors can be generally attributed to being the result of either (1) the imprecise positioning of the driveshaft assemblies on the balancing machine, (2) the internal operation of the balancing machine itself, and/or (3) part variation (e.g., looseness of the components within a part can cause inconsistent measurements of imbalance). Because of the measurement errors that are generated as a result of the balancing machine discussed above, it is common for each driveshaft to be rotated and measured more than two times in order to achieve an adequate level of confidence that the proper balance has been achieved. This time consuming and repetitious process is particularly problematic in the context of balancing vehicular driveshaft assemblies, which are typically manufactured in relatively large volumes.
A variety of attempts have been made to account for these measurement errors and thereby improve productivity and quality in the balancing process. However, known attempts to account for these measurement errors have met with limited success.
One aspect of these attempts is to monitor production gage R & R (Repeatability and Reliability), that is, to monitor how well the balance machines, together with the fixtures for mounting the driveshafts on the balance machines produce repeatable and reliable results. In the past, long term gage R & R studies involved the repeated measurement of one or more known driveshaft test samples. During this long term study, the balancing machine would obviously be unavailable for use in measuring production throughput, impacting productivity. It would be desirable to provide an improved method for statistically analyzing the operation of a machine for balancing articles for rotation so as to reduce measurement errors and thereby enhance productivity and quality.